1. Field of the Invention
This invention relates to an ultrasonic probe used in an ultrasonic imaging device or the like, and more particularly to an ultrasonic probe constituted by a multilayer piezoelectric material.
2. Description of the Related Art
The following patent disclosures which explain the related art can be given:
(1) Japanese Patent Disclosure (Koukai) No. 60-41399; and
(2) Japanese Patent Disclosure (Koukai) No. 61-69298.
The ultrasonic probe is constructed mainly by a piezoelectric element which is used to obtain image data indicating the internal state of an object by receiving ultrasonic waves reflected from the interface in the object having a different acoustic impedance when ultrasonic waves are applied to the object. For example, an ultrasonic diagnostic apparatus for examining the internal portion of a human body and an inspecting apparatus for searching for scars occurring in the internal portion of welded metal may be given as concrete examples of the ultrasonic imaging apparatus using the above ultrasonic probe.
In the ultrasonic diagnostic apparatus, it is required to obtain high-resolution images with a high sensitivity so that a cavity (gap) which is caused by the small physical variation due to variation in the condition of a patient can be clearly observed. It is considered to increase the number of elements of a transducer or raising the resonant frequency thereof as a method for attaining the high-resolution required for the ultrasonic probe.
In a case where the number of elements of the transducer used in the ultrasonic probe is increased to attain the above purpose, the resolution in a direction parallel to the array of the transducer elements can be enhanced. At the same time, the ultrasonic wave radiation area for each transducer element is reduced and the impedance of each transducer element is increased. In particular, the ultrasonic wave radiation area of each transducer element in an electronic sector scanning probe for effecting the sector-scanning operation by supplying driving signals to a plurality of strip-form transducer elements with a time delay may be reduced to 1/2 to 1/5 of that obtained in a linear scanning probe having the same construction and effecting the linear scanning operation, and therefore, the impedance of each transducer element is increased more significantly. As a result, the voltage loss caused in the sector scanning probe by the presence of the electrostatic capacitance of a coaxial cable connecting the probe head to the main section of the device becomes larger in comparison with that of the linear scanning probe.
In a case where the resonant frequency used in the ultrasonic probe is increased to attain the above purpose, it must be considered that, in recent years, it has been required to observe intraepidermal tissue or internal body tissue of a patient under operation as an image with a high resolution. In order to meet the requirements, the frequency is set in the range of 15 to 30 MHz. However, since the ultrasonic probe generally utilizes the thickness expander mode of the piezoelectric element, it is necessary to make the piezoelectric element thin in order to attain the high frequency operation. This problem becomes more severe in ultrasonic probes using a multilayer piezoelectric material disclosed in Japanese Patent Disclosure No. 61-69298, for example. That is, in the multilayer piezoelectric material disclosed in the above Japanese Patent Disclosure, since piezoelectric layers are electrically connected in parallel, a resonance occurs at a frequency of the ultrasonic wave set when the total thickness of the multilayer piezoelectric material (total thickness of a plurality of laminated piezo electrodes) becomes equal to half the wavelength thereof. Therefore, in electric material must be formed as thin as possible.
In general the piezoelectric element may be roughly divided into two types; piezoelectric ceramic and high-polymer piezoelectric element.
In the case of piezoelectric ceramic, the thickness of the piezoelectric element is less than 100 .mu.m. In the extremely thin piezoelectric element, and particularly, in the case of using ceramic such as PZT-series ceramic containing lead, the characteristic of the ceramic is largely influenced by lead diffused into the sintering atmosphere in the sintering process. As a result, the characteristic of the ceramic is degraded, the piezoelectric element itself may be warped, and at the same time, the workability thereof becomes lowered. Further, in most of the ordinary piezoelectric elements, sintered electrodes of silver or the like are bonded thereto, and in this case, printing electrode paste containing glass frit for closely joining silver and ceramic is used so that the ratio of the glass frit diffused into the ceramic may increase with a decrease in the thickness of the ceramic. As a result, the characteristic of the piezoelectric element itself may be degraded.
In the case of high-polymer piezoelectric element, the piezoelectric element is soft in comparison with the piezoelectric ceramic and may be less damaged. However, it has the following defects. That is, the electromechanical coupling factor thereof is as small as 0.2 to 0.3. The dielectric constant thereof is smaller by more than two digits in comparison with that of ceramic. The glass transition temperature thereof is as low as approx. 100.degree. C. Therefore, the high-polymer piezoelectric element is not generally used as an array probe.
As described above, the two types of piezoelectric elements have defects from the view points of material, shape and the like.
The following three methods for obtaining images at a high sensitivity by use of the ultrasonic probe are given:
(1) increase the electromechanical coupling factor of the piezoelectric element;
(2) obtain the acoustic matching; and
(3) obtain the electrical matching.
The maximum value of k'.sub.33 of the currently available piezoelectric ceramic material which can be used to effect the above method (1) is approx. 0.7. Much effort has been made to increase the electromechanical coupling factor, but optimum material as the piezoelectric element better than lead zirconate titanate-series ceramic represented as PZT developed by Clevite Co. in 1955 has not been developed.
In order to effect the method (2), the difference of the acoustic impedance between the piezoelectric element and the living body becomes large and therefore a method for forming an acoustic matching layer is used. The number of acoustic matching layers may be set to one, or more than one, but the improvement over the piezoelectric element currently used cannot be expected only by using the acoustic matching layer.
Various methods are used to effect the method (3). In the ultrasonic diagnostic apparatus, the number of elements of the ultrasonic probe tends to increase because of the high-resolution required in recent years. Therefore, the ultrasonic wave radiation area for each element becomes small and the impedance thereof becomes large. As a result, the voltage loss due to the presence of the electrostatic capacitance of the coaxial cable becomes larger as described before.
Further, the electronic sector scanning probe is not only used in the operation of photographing B mode images which are the tomographic images of the living body, but also often used in the photographing operation in the Doppler mode in which the blood flow rate in the heart, liver, carotid artery or the like is displayed in color by making use of the Doppler shift (Doppler effect) of the ultrasonic waves caused by the blood flow therein. In the case of the Doppler mode, since the reflected echo from fine corpuscles with the diameter of several .mu.m is used, the level of a signal obtained is low in comparison with the case of the above-described B mode. Therefore, the sensitivity margin in the Doppler mode is small in comparison with the case of the B mode and it is necessary to further enhance the sensitivity.
Recently, a "color flow mapping (CFM) method" for two-dimensionally mapping the diffusion of blood flow on the real time base and color-displaying the flow and reflection power of the blood flow is widely used, and therefore the diagnostic function and the diagnostic application field are significantly enlarged. The CFM method is used for the diagnostic of various organs of a human body such as the uterus, kidney and pancreas. Now, the research and development of the diagnostic apparatus for making it possible to observe the movement of coronary blood flow are made in various hospitals and research laboratories.
It will be understood difficult from consideration of the inherent property of the probe to observe the weak blood flow such as coronary blood flow and variation in the blood flow caused by hyperplasia of early cancerous cells. In order to solve the above problem, probe heads which are improved to reduce the loss caused by the electrostatic capacitance of the coaxial cable by inserting an emitter follower circuit used as an impedance transducer between the probe head and the coaxial cable are practically used. However, even with this type of probe, it is difficult to observe the weak blood flow described before.
When the improvement of an ultrasonic diagnostic apparatus is considered, it is possible to enhance the sensitivity thereof by raising the driving voltage supplied to the probe head. However, since the electric power supplied to the piezoelectric element is also increased, heat caused by the dielectric loss and ultrasonic power irradiated to acoustic lens or backing material may be generated and the generated heat may degrade the characteristic of the probe or give damage such as a burn to the human body. Therefore, increase in the driving voltage is limited, and the sensitivity cannot be sufficiently enhanced only by the improvement made by the above method.
In addition to the improvement made by the above method, the following improvements are further developed. In general, the reference frequency in the Doppler mode is set lower than the center frequency of the frequency bandwidth of the ultrasonic probe. The reason for this is that it is preferable to us low frequency ultrasonic waves in order to suppress the influence by reduction in the S/N ratio due to attenuation of the ultrasonic waves in the living body. Therefore, if ultrasonic waves having two types of frequency components can be transmitted/received by a single ultrasonic probe, it becomes possible to obtain the B mode image of high resolution in the high frequency components and the Doppler image of high sensitivity in the low frequency components. In order to realize such a device, "duplex type ultrasonic probes" in each of which two types of transducers having different resonant frequencies are provided in a single ultrasonic probe head are manufactured and sold from various makers. However, since this type of ultrasonic probe has a plurality of transducers having different resonant frequencies, the ultrasonic wave transmission and reception planes are set in different positions, making it impossible to observe the same tomographic image.
Therefore there is proposed a device which can transmit/receive ultrasonic waves having two different types of frequency bands by means of a single transducer and which is formed by using a multilayer piezoelectric material constructed as is disclosed in Japanese Patent Disclosure No. 60-41399. That is, the two types of frequency bandwidths can be separated by use of a combination of the ultrasonic probe, a driving pulse width and a filter, and as a result, the B mode signal and Doppler signal can be separately obtained by use of the high-frequency components and low-frequency components, respectively. However, even with the ultrasonic probe of the above construction, since the electromechanical coupling factor of a single piezoelectric element is substantially equally divided, the frequency band on the high-frequency side becomes narrow and the tailing remaining of the echo signal is lengthened. As a result, the high resolution cannot be enhanced to an expected value even when attempt is made to obtain a B mode image of high resolution by the high frequency components. Further, since the low frequency components tend to be reduced as the frequency band becomes narrower, the S/N ratio thereof is lowered, thus causing insufficient penetration. The reason is that the frequency component of an echo signal from the deep portion of the living body is constituted by components of frequencies lower than the center frequency of the transmitted ultrasonic waves. The specific frequency bandwidth required for obtaining preferable B mode images is more than 40% of the center frequency. For example, the specific bandwidth at -6 dB is 40 to 50% in the case of a single-layered matching and 60 to 70% in the case of two-layered matching when a piezoelectric element of single layer structure is used. In contrast, when the piezoelectric element of the above construction is used, the specific bandwidth is 25% of the center frequency in the case of a single-layered matching and 35% in the case of two-layered matching. Thus, the specific bandwidth which is only half that obtained when the conventional single-layered piezoelectric element is used can be obtained, and therefore further improvement must be made in this respect.
As described above, when the piezoelectric ceramic is used in the conventional technology for setting the frequency high by reducing the thickness of the piezoelectric element so as to attain an ultrasonic probe of high resolution, the thickness must be made extremely thin. Therefore, problems occur from the view points of manufacturing method and characteristic thereof. Further, the high-polymer piezoelectric element cannot be practically used because of the small electrode mechanical coupling factor thereof.
In the electronic sector scanning probe often used in the Doppler mode, it cannot be expected to significantly enhance the sensitivity by properly selecting the material of the piezoelectric element and disposing an acoustic matching layer. It is pointed out that the sensitivity is not so high even in the probe head in which the voltage loss caused by the electrostatic capacitance of the cable itself is reduced by inserting the emitter follower circuit between the probe and the coaxial cable.
Further, the method for enhancing the sensitivity by raising the driving voltage is restricted by the problem of heat generation in the piezoelectric element. Also, in a case where two different frequency bandwidths are obtained by using a single ultrasonic probe, there is provided a problem that the same portion cannot be observed when a plurality of transducers having different resonant frequencies are used. Further, a multilayer piezoelectric material which is proposed to solve the above problem and is formed by laminating piezoelectric elements having substantially the same thickness as the single-layered piezoelectric element disclosed in Japanese Patent Disclosure No. 60-41399 has a problem that the specific frequency bandwidth of the high-frequency components is narrow.